This process has become known as "stereolithography". It can be used for example for producing a prototype of a part from a computer aided design drawing directly, i.e. without mechanical work. A preferred form of this process is described in U.S. Pat. No. 4,575,330. The three-dimensional object is produced in a body of liquid medium capable of photopolymerization. A programmed movable spot of UV light shining on the surface of the liquid causes the photopolymerizable medium to become polymerized at the point of incidence. By repeatedly moving the spot of light forwards and backwards it is possible to produce a two-dimensional laminar polymer layer composed of a multiplicity of individual points. Then the platform on which this polymer layer rests is moved down and new liquid flows across the surface, and another polymer layer can be produced by specific irradiation. The polymer layers are so adhesive that they become joined together in the liquid. When finally after multiple repetition of these steps the object is formed, it is removed from the liquid, washed and completely cured in a separate operation.
Recently a stereolithography process of this type was put into industrial practice using a laser light source. However, despite the use of laser beams, the process is very time-consuming, since a multiplicity of individual points must be irradiated and the laser beam cannot be moved onto the next point until the polymerization has progressed to a sufficient extent at the preceding point, which takes a few seconds. Despite this long polymerization time, the object still needs to be fully cured in a separate operation. Moreover, the use of laser beams is very costly.